A mutation in the lacZ gene alters β-galactosidase so it makes more allolactose compared with a normal β-galactosidase enzyme. How would this mutation affect the expression of the lac operon?

Question

BlackTom AI · Accepted Answer

Allolactose is the natural inducer of the lac operon, so increased production of allolactose by a mutant β-galactosidase will lower the external lactose threshold needed to derepress the operon.

1) Relevant knowledge points and concepts
- The lac operon in E. coli is negatively regulated by the LacI repressor, which binds the operator to block transcription.
- Allolactose (a lactose isomer) is the inducer: it binds LacI, causing a conformational change that prevents LacI binding the operator, allowing transcription of lacZYA.
- β-galactosidase (product of lacZ) catalyzes lactose hydrolysis and also produces allolactose via a transglycosylation side reaction. This creates a positive feedback loop: some LacZ activity generates allolactose, which derepresses the operon and increases lacZ expression.
- Induction depends on the intracellular concentration of allolactose, which is influenced by extracellular lactose and β-galactosidase activity.

2) Step-by-step reasoning
- Normal scenario: external lactose enters the cell slowly; a small basal level of lacZ expression produces β-galactosidase that converts some lactose to allolactose. When intracellular allolactose reaches a sufficient level, LacI is inactivated and full induction occurs.
- Mutant scenario: β-galactosidase produces more allolactose per unit lactose. For the same low external lactose concentration, the intracellular allolactose will be higher than normal.
- Higher intracellular allolactose more readily binds LacI and causes derepression at lower external lactose concentrations.

3) Calculations
- No numerical calculation is required; conceptually, if allolactose production per molecule of lactose increases, then the external lactose concentration required to reach the induction threshold decreases.

4) Why the correct answer is correct
- Because more allolactose is generated from each lactose molecule, the inducer threshold for derepression is reached at lower environmental lactose levels. Thus less lactose is required to induce the lac operon.

5) Why other options are incorrect
- "Could not be expressed in the presence of lactose": false — increased allolactose promotes expression, not prevents it.
- "Constitutively expressed in the absence of lactose": false — without lactose substrate, even a more active enzyme cannot generate allolactose, so LacI remains bound and operon remains off (except for normal leaky expression).
- "Would require more lactose": false — the mutation increases inducer production per lactose, so it reduces, not increases, the required lactose.

6) Conclusion
This reasoning confirms the provided correct answer: It would require less lactose in the environment to express the lac operon.

A mutation in the lacZ gene alters β-galactosidase so it makes more allolactose compared with a normal β-galactosidase enzyme. How would this mutation affect the expression of the lac operon?单项选择题

类似问题

How does the catabolite repression mechanism, involving the cAMP-CAP complex, modulate the expression of the lac operon when both glucose and lactose are present?

In the presence of lactose but absence of glucose, what happens to cAMP levels and the lac operon?

A researcher grows E. coli in minimal medium containing both glucose and lactose and observes that the lac operon is not highly expressed. Which explanation best accounts for this observation?

The binding of ___________ to the lac repressor disables the lac repressor to bind to the site, thereby transcription.

On the chromosome, CAP is mutated such that it no longer produces a functional CAP protein, and the operator is mutated such that it no longer binds to the repressor. On the plasmid, lacI is mutated so that it no longer produces a functional repressor protein. Lactose and glucose are both absent.

Match the following states of nutrient availability with the corresponding state of the lac operon during gene expression regulation: 1: High glucose present, no lactose present 2: High glucose present, lactose also present 3: Low glucose present, high lactose present

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